Electric Power Steering (EPS) systems utilize an electric motor, typically a Permanent Magnet Synchronous Machine (PMSM), to provide steering assist to the driver. PMSMs are controlled using a well-known technique called Field Oriented Control (FOC), which allows sinusoidal three phase motor voltage and current signals to be transformed into equivalent signals in a synchronously rotating reference frame, commonly referred to as the DQ reference frame, where the motor voltages and currents become DC quantities. The reference frame transformation requires information of the absolute position of the rotor of the machine, which is usually obtained by using a physical position sensor. Sensorless Control (SC) refers to the class of control schemes in which the motor is controlled without a physical position sensor. SC methods can be classified into two broad categories, namely the Direct Torque Control (DTC) and Field Oriented Control (FOC) methods. While DTC does not require any direct knowledge of the position, FOC relies on direct knowledge of position. In contrast with their sensor based counterpart, SC-FOC schemes employ observers or estimators to obtain the position of the machine, however, the overall control architecture remains identical to a sensor based FOC scheme. EPS systems typically employ a sensor based FOC system for motor control, but SC provides an alternative to provide Fault Tolerant Control (FTC) in order to avoid Loss of Assist (LoA) when the position sensor fails.
Most SC algorithms require estimation of the stator flux linkages for the control implementation. For DTC systems, the flux estimation is used for torque estimation, which is then used along with the flux estimates to generate gate signals for the Voltage Source Inverter (VSI) directly. FOC systems require flux estimates in order to estimate the machine velocity and position. For high performance applications such as EPS, it is critical to accurately estimate the stator flux, because errors in these quantities result in direct performance degradation of the overall drive system.